Absorbent polymer having reduced dusting tendencies

ABSTRACT

PCT No. PCT/US93/02872 Sec. 371 Date Oct. 31, 1995 Sec. 102(e) Date Oct. 31, 1995 PCT Filed Mar. 29, 1993 PCT Pub. No. WO94/22940 PCT Pub. Date Oct. 13, 1994The invention provides water-swellable polymer compositions having reduced dusting tendencies, i.e, reduced amounts of unassoiated dust having a maximum diameter less than or equal to 10 microns and/or the tendency to generate reduced amounts of unassociatcd dust having a maximum diameter less than or equal to 10 microns upon attrition, and a process for preparing such compositions.

BACKGROUND OF THE INVENTION

The subject invention pertains to an absorbent polymer having reducedlevels of unassociated dust, to an attrition-resistant absorbent polymerand to a process for the preparation of such polymers, and to a processfor determining the amount of unassociated matter having a diameter lessthan or equal to a predetermined size.

In the gel polymerization of water-swellable polymers, monomers arepolymerized in aqueous solution. Certain additives, such as crosslinkingagents, may be incorporated into the monomer mixture. The product of thepolymerization process is typically dried and subjected to mechanicalmeans of particle size reduction and classification including chopping,grinding, and sieving. During such particle size reduction, ultrafineparticles or dust, i.e., particles having a mean diameter less thanabout 10 microns, are unavoidably created.

Japanese Patent Application 6200745-A discloses a water-swellable resincomposition having low levels of dust prepared bytreating the polymerwith an aqueous solution of a salt and/or hydroxide of a polyvalentmetal, the salt and/or hydroxide of the polyvalent metal being providedin the amount of 0.01 to 10 weight percent, based on the weight of thedried polymer.

European Patent Application 103,058 discloses a water-absorbent materialconsisting of an intimate mixture of 5 to 95 weight percent particulatehydrogel material and 95 to 5 weight percent filler material, whichfiller material has been treated with up to 1 weight percent of asurfactant. The disclosed materials are said to have reduced dustingtendencies. The disclosed technology, however, would not lead todecreased dusting or the decreased attrition at the site of themanufacture of the hydrogel or during subsequent transport and handlingof the hydrogel.

U.S. Pat. No. 4,080,358 discloses a low dusting, free-flowingcomposition comprising a mixture of a water-soluble acrylamide polymerprimarily of a particle size less than 0.5 mm, from 0.1 to 0.5 weightpercent based on the weight of the acrylamide of an antidusting agent,e.g., a propylene oxide adduct of glycerol of an average molecularweight between 500 and 2000, and from 1 to 80 weight percent of a highlysilicious free-flow aid of submicron to 10 micron particle size.

U.S. Pat. No. 4,734,478 discloses a water-swellable polymer wherein themolecular chains near the surface of the particles are crosslinked. Thepolymer is prepared by mixing 100 parts water-swellable powder with from0.001 to 10 parts polyhydric alcohol, and heating the mixture to 90° C.or more to react them. The surface crosslinked particles are said tohave no decrease in flowability.

Industry would find great advantage in an aqueous fluid absorbentmaterial which does not dust, e.g., which comprises no more than 2.5 ppmunassociated particles less than 10 microns in diameter. Industry wouldfurther find advantage in an attrition-resistant aqueous fluid absorbentmaterial, e.g., which comprises no more than 35 ppm unassociated dustupon roll-milling for 20 minutes, and/or which comprises no more than 15ppm unassociated dust upon roll-milling for 10 minutes. Industrywouldfurther find advantage in a process for reducing the level of particlesless than 10 microns in diameter in a given composition by at least 90percent. Industry would further find advantage in a process forascertaining the amount of unassociated dust in a water-swellablepolymer composition.

SUMMARY OF THE INVENTION

Accordingly, the subject invention provides a composition comprisingwater-swellable gel-polymerized polymer particles, the particles bearingon their surface a physical coating of a dedusting agent, wherein thecomposition comprises no more than 2.5 ppm unassociated dust having amaximum diameter less than or equal to 10 microns and wherein thecomposition is characterized by the ability to absorb at least 20 gramsof a 0.9 weight percent aqueous saline solution under a pressure of 0.3psi (21,000 dynes/cm²), i.e., a 60 minute 0.3 psi (21,000 dynes/cm²) AULgreater than 20 grams/gram as measured in accordance with the AbsorptionUnder Load Test set forth in U.S. Pat. No. 5,147,343, incorporatedherein by reference.

The subject invention further provides an attrition-resistantcomposition comprising water-swellable gel-polymerized polymerparticles, the particles bearing on their surface a physical coating ofa dedusting agent, wherein the composition is characterized by a 60minute 0.3 psi (21,000 dynes/cm²) AUL greater than 20 grams/gramcomprises no more than 35 ppm unassociated dust having a maximumdiameter less than or equal to 10 microns upon being roll milled for 20minutes and/or which comprises no more than 15 ppm unassociated dusthaving a maximum diameter less than or equal to 10 microns upon beingroll milled for 10 minutes.

The subject invention further provides a process comprising:

(a) preparing a water-swellable hydrogel by a gel polymerizationprocess;

(b) drying and sizing the hydrogel to form a composition comprisingdried and sized particles, the composition comprising a first amount ofunassociated dust having a maximum diameter less than or equal to 10microns; and

(c) contacting the composition with a dedusting agent to form a dedustedcomposition, wherein the dedusted composition is characterized by a 60minute 0.3 psi (21,000 dynes/cm²) AUL greater than 20 grams/gramwhereupon the dedusted composition comprises a second amount ofunassociated dust having a maximum diameter less than or equal to 10microns, the second amount being less than about 10 percent of the firstamount.

The subject invention further provides a process comprising:

(a) preparing a water-swellable hydrogel by a gel polymerizationprocess;

(b) drying and sizing the hydrogel to form a composition comprisingdried and sized particles, the composition comprising a first amount ofunassociated dust having a maximum diameter less than or equal to 10microns; and

(c) contacting the composition with a dedusting agent to form anattrition resistant composition characterized by a 60 minute 0.3 psi(21,000 dynes/cm²) AUL greater than 20 grams/gram and by a tendency togenerate a second amount of unassociated dust having a maximum diameterless than or equal to 10 microns upon roll milling of the attritionresistant composition for 10 minutes, the second amount being less thanor equal to the first amount.

The subject invention further provides a process for measuring theamount of unassociated matter having a diameter less than apredetermined size present in a water-swellable polymer compositioncomprising:

(a) placing the composition into the sample holder of a pulsed jetdisperser;

(b) blasting the composition with air dried by passage throughdehydration means; and

(c) determining the weight of particles in each of a plurality of sizeincrements.

DETAILED DESCRIPTION OF THE INVENTION

As used herein, the term "unassociated dust" means the portion of awater-absorbent polymer particle composition having a maximum diameterless than or equal to 10 microns, which portion is rendered airbornewhen a blast of air is applied to the composition, e.g., as occursduring the measurement of unassociated dust using an AEROSIZER™ pulsedjet disperser described below.

As used herein the term "physical coating" means that the dedustingcomposition is in contact with both the polymer particles and dust,without chemically reacting with either the particles or the dust, suchas through a surface crosslinking reaction.

The water-swellable or lightly crosslinked hydrophilic polymers that areusefully used in the present invention can be any of the knownhydrophilic polymers which are capable of absorbing large quantities offluids. In particular, water-absorbent polymers useful in this inventionare water-absorbent polymers which contain carboxyl moieties.Preferably, at least about 0.01 equivalent of carboxyl groups arepresent per 100 grams of the water-absorbent resin.

Among preferred carboxyl-containing water absorbent polymers arehydrolyzates of starch-acrylonitrile graft copolymers, partiallyneutralized products of starchacrylic acid graft copolymers,saponification products of vinyl acetate acrylic ester copolymers,hydrolyzates of acrylonitrile copolymers, crosslinked products ofhydrolyzates of acrylonitrile copolymers, hydrolyzates of acrylamidecopolymers, crosslinked products of hydrolyzates of acrylamidecopolymers, partially neutralized products of polyacrylic acids andcrosslinked products of partially neutralized polyacrylic acids.

Examples of some suitable polymers and processes for preparing them aredisclosed in U.S. Pat. Nos. 3,997,484; 3,926,891; 3,935,099; 4,090,013;4,093,776; 4,340,706; 4,446,261; 4,683,274; 4,459,396; 4,708,997;4,076,663; and 4,190,562, the relevant portions of which areincorporated herein by reference. Such hydrophilic polymers are preparedfrom water-soluble α,β-ethylenically unsaturated monomers such asmonocarboxylic acids, polycarboxylic acids, acrylamide and theirderivatives.

Suitable α,β-ethylenically unsaturated monomers include, for example,acrylic acid, methacrylic acid, crotonic acid, isocrotonic acid andalkali metal salts and ammonium salts thereof; maleic acid, fumaricacid, itaconic acid, acrylamide, methacrylamide and2-acrylamido-2-methyl-1-propane sulfonic acid and its salts. Thepreferred monomers include acrylic acid and methacrylic acid and theirrespective salt forms such as alkali metal or ammonium salts.

The water-soluble monomers useful in the present invention may be usedin amounts ranging from about 10 percent to about 80 percent by weightbased on the total weight of the aqueous monomer solution. Preferably,the amount ranges from about 20 percent to about 60 percent based on thetotal weight of the aqueous monomer solution.

Optionally, the minor amounts of other water-soluble, unsaturatedmonomers, such as alkyl esters of the acid monomers, e.g., methylacrylate or methyl methacrylate may be present. In addition, certaingrafting polymers, such as, for example, polyvinyl alcohol, starch andwater soluble or swellable cellulose ethers may be employed to prepareproducts having superior properties. Such grafting polymers, whenemployed, are used in amounts upto about 10 weight percent based on theα,β-ethylenically unsaturated monomer. Further, it may be advantageousto include a chelating agent to remove trace metals from solution, e.g.,when a metal reaction vessel is employed. One such chelating agent isVERSENEX™ V-80 (an aqueous solution of the pentasodium salt ofdiethylenetriamine pentacetic acid) (Trademark of The Dow ChemicalCompany). Such chelating agents, when employed, are generally used inamounts between about 100 and about 2000 ppm based on theα,β-ethylenically unsaturated monomer.

It is desirable to obtain a level of conversion of monomer to polymer ofat least about 95 percent. The polymerization may be carried out usingacid monomers that are not neutralized or that have been neutralized orpartially neutralized prior to the polymerization. Neutralization isconveniently achieved by contacting the aqueous monomer with an amountof basic material sufficient to neutralize between about 20 and about 95percent of the acid groups present in the acid monomers. Preferably, theamount of basic material will be sufficient to neutralize between about40 percent and 85 percent, and most preferably between about 55 percentand about 75 percent of the acid groups present in the acid monomers.When pre-neutralizing the monomer solution, it is important to controlthe neutralization conditions so that the heat of neutralization doesnot cause the premature polymerization of the monomer mixture. Theneutralization is advantageously carried out at temperatures below about40° C., preferably at temperatures below about 35° C.

Compounds which are useful to neutralize the acid groups of the monomerare typically those which will sufficiently neutralize the acid groupswithout having a detrimental effect on the polymerization process. Suchcompounds include alkali metal hydroxides, and alkali metal carbonatesand bicarbonates. Preferably, the material used to neutralize themonomer is sodium or potassium hydroxide or sodium carbonate orpotassium carbonate. In determining the desired degree ofneutralization, care must be taken to ensure that the pH of theresulting crosslinked absorbent polymer, which will be contacted with ordispersed in an aqueous fluid to be absorbed, is maintained in a rangeappropriate for the applications for which the polymer is intended.Alternatively, the polymerization may be carried out employingunneutralized monomers and thereafter neutralizing, as is known in theart.

Conveniently, a conventional vinyl addition polymerization initiator isused in the polymerization of the water-soluble monomers and thecrosslinking agent. A free radical polymerization initiator which issufficiently soluble in the monomer solution to initiate polymerizationis preferred. For example, water soluble persulfates such as potassiumpersulfate, ammonium persulfate, sodium persulfate, and otheralkali-metal persulfates, hydrogen peroxide and water solubleazo-compounds such as 2,2'-azobis (2-amidinopropane HCI) may be used.Some of these initiators, such as hydrogen peroxide, can be combinedwith reducing substances such as sulfites or amines to form known redoxtype initiators. The total amount of initiators used may range fromabout 0.01 to about 1.0 weight percent, preferably about 0.01 to about0.5 weight percent, based on the total weight of α,β-ethylenicallyunsaturated monomer reactants.

The water-absorbent resin will preferably be lightly crosslinked torender it water-insoluble. The desired crosslinked structure may beobtained by the copolymerization of the selected water-soluble monomerand a crosslinking agent possessing at least two polymerizable doublebonds in the molecular unit. The crosslinking agent is present in anamount effective to crosslink the water-soluble polymer. The preferredamount of crosslinking agent is determined by the desired degree ofabsorption capacity and the desired strength to retain the absorbedfluid, i.e., the desired absorption under load (AUL). Typically, thecrosslinking agent is used in amounts ranging from about 0.0005 to about5 parts by weight per 100 parts by weight of α,β-ethyIenicallyunsaturated monomer used. More preferably, the amount ranges from about0.1 to about 1 part by weight per 100 parts by weight of theα,β-ethylenically unsaturated monomer. If an amount over about 5 partsby weight of crosslinking agent per 100 parts is used, the resultingpolymer has too high a crosslinking density and exhibits a reducedabsorption capacity and increased strength to retain the absorbed fluid.If the crosslinking agent is used in an amount less than about 0.0005part by weight per 100 parts, the polymer has too low a crosslinkingdensity, and when contacted with the fluid to be absorbed becomes stickyand exhibits a lower initial absorption rate.

While the crosslinking agent will typically be soluble in the aqueoussolution of the α,β-ethylenically unsaturated monomer, the crosslinkingagent may be merely dispersible in such a solution, without negativeimplications. The use of such dispersing agents is disclosed in U.S.Pat. No. 4,833,222, the relevant portions of which are incorporatedherein by reference. Suitable dispersing agents include carboxymethylcellulose suspending aids, methyl cellulose, hydroxypropyl cellulose,and polyvinyl alcohol. Such dispersing agentsare typically provided at aconcentration between about 0.005 and about 0.1 weight percent, based onthe total weight of α,β-ethylenically unsaturated monomer reactants.

Typical crosslinking agents include monomers having in one molecule 2 to4 groups selected from the group consisting of CH₂ ═CHCO--, CH₂═C(CH₃)CO-- and CH₂ ═CH--CH₂ --. Exemplary crosslinking agents arediacrylates and dimethacrylates of ethylene glycol, diethylene glycol,triethylene glycol, propylene glycol, 1,4-butanediol, 1,5-pentanediol,1,6-hexanediol, neopentyl glycol, trimethylolpropane andpentaerythritol; triacrylates and trimethacrylates of trimethylolpropaneand pentaerythritol; highly ethoxylated trimethylol propane triacrylate;tetracrylate and tetramethacrylate of pentaerythritol; andtetraallyloxyethane.

As noted in W093/05080, the relevant portions of which are incorporatedherein by reference, a certain class of crosslinking agents yieldsparticularly preferred absorptive properties. Such preferredcrosslinking agents include methylenebisacrylamide,bis(acrylamido)acetic acid and its salts, allyl acrylate, allylmethacrylate, and esters or amides having both a vinyl and an allylfunctionality.

In a preferred embodiment for making polymers useful in the practice ofthis invention, an aqueous solution of the α,β-ethylenically unsaturatedmonomer in the partially neutralized form, the crosslinking agent, theinitiator and a grafting polymer substrate, if desired, is prepared.

The polymerization of the mixture may be initiated by elevating thetemperature of the mixture containing the initiator or by using aredox-type initiator as described above. Generally, the temperature atwhich polymerization will begin ranges from about 20° C. to about 45° C.The temperature at which the polymerization is carried out is highlydependent on the type of monomers used and the specific initiator systememployed. Preferably, the maximum temperature of polymerization rangesfrom about 50° C. to about 100° C., most preferably from about 60° C. toabout 100° C. The method by which the temperature of the polymerizationis controlled is not critical so long as sufficient cooling is presentto remove the heat which is generated during the polymerization.

The resultant polymer is typically pre-sized and dried using meanswell-known in the art. Suitable drying means include fluidized beddriers, rotary driers, forced air ovens, through circulation banddriers, etc. In some instances, drying will occur in two or more stages.In two-stage drying, the pre-sized polymer particles are partially driedin the first stage, e.g., the pre-sized polymer particles are dried toless than about 10 percent moisture level, preferably about a 5 percentmoisture level. During initial drying, the pre-sized particles typicallyfuse together into sheets. Following the completion of drying, thepolymer is more completely sized to form particles having an averagediameter less than about 0.8 mm. During such sizing, dust, characterizedby extremely small particle sizes may result, i.e., particle sizes lessthan or equal to 10 microns.

To improve absorptive properties, the dried particles may be heattreated in accordance with the procedures set forth in WO93/05080,and/or PCT Application PCT/US92/07611 filed on Sep. 9, 1992 the relevantportions of which are incorporated herein by reference. In particular,the dried particles are heated for a time sufficient to increase themodulus, centrifuge capacity, and/or the absorbency under load. Suchheat treatment is preferably carried out at a temperature of at leastabout 170, more preferably of at least 180, and most preferably of atleast about 190° C. Such heat treatment is preferably carried out at atemperature of less than about 250, more preferably less than about 240°C.

The time period for heat treatment should be sufficient to effect animprovement in absorptive properties. The exact times of heat treatmentrequired will be affected by the equipment chosen, and can be determinedempirically by examination of product properties. Preferably, the timeis at least about 3 minutes, and more preferably at least about 5minutes. If the time is too long, the process becomes uneconomical and arisk is run that the absorbent resin may be damaged. Preferably, themaximum time of heating is about 150 minutes or less, more preferably 60minutes or less.

The method of heat treatment is not critical. For example, forced airovens, fluidized bed heaters, heated screw conveyors, and the like maybe successfully employed. If desired, the heated polymer may beremoisturized for ease in handling. While such remoisturization mayserve to decrease the amount of unassociated dust, it may lead toclumping of the polymer product.

To reduce levels of unassociated dust and to inhibit the production ofunassociated dust during handling of the polymer material, the dried andoptionally heat treated particles are contacted with an effective amountof a dedusting agent. The dedusting agent will serve to adhere the dustto the larger polymer particles or to the walls of the mixing vessel orcontainer in which the polymer is retained during handling, which willtranslate to reduced levels of unassociated dust in the finished polymerproduct at the various stages of handling. Moreover, the application ofthe dedusting agent to the polymer samples does not appear todetrimentally affect the performance or properties of the polymer, andfurther appears to reduce the tendency of the polymer to maintain astatic charge.

As defined herein, the term "dedusting agent" means a material which,when applied to the polymer material, reduces the concentration ofunassociated dust having a diameter of less than 10 microns by at least90 weight percent, more preferably by at least 95 percent, and mostpreferably by at least 99 percent and/or which produces dedustedcompositions comprising less than 2.5 ppm unassociated dust having amaximum diameter less than or equal to 10 microns.

Preferred dedusting agents will be hydrophilic organic materials whichdo not substantially penetrate into the interior of the polymerparticles. Suitable dedusting agents will be hydrophilic, whereby waterabsorption by the coated particles is not impeded by the presence of thededusting agent. Suitable dedusting agents will further be substantiallynonpenetrating, such that the dedusting agent is present at the surfaceof the particles, even when provided in small amounts.

Exemplary dedusting agents include water-soluble, lower aliphaticpolyols of greater than about 200 average molecular weight or lowerpolyalkylene glycols of about 400 to about 6000 average molecularweight. One preferred dedusting agent is a propylene oxide adduct ofglycerol or a polyethylene glycol having an average molecular weight ofabout 500 to about 2000. In particular, exemplary dedusting agentsinclude VORANOL™ polyether polyols such as VORANOL™ 2070, VORANOL™ 2100,and VORANOL™ 3100 polyether polyols (all available from The Dow ChemicalCompany); polyethylene glycols; glycerin; polyether alcohols, such asDOWANOL™ TPM tripropylene glycol methyl ether, DOWANOL™ TPnBtripropylene glycol n-butyl ether, and DOWANOL™ PPH propylene glycolphenyl ether (available from The Dow Chemical Company); TRITON™ X-100surfactant (available from Union Carbide); TERGITOL™ 15-S-9 ethoxylatedsurfactant (available from Union Carbide); and nonionic surfactantshaving an HLB value of at least 7. Although it is not necessary toinclude water or solvents in the addition of the hydrophlic treatment tothe dry polymer powder, amounts of water or solvents upto 20 times theamount of the hydrophilic treatment may be employed. When an organicsolvent is employed, it will be preferable to utilize a solvent which isreadily removed from the coated particles, e.g., by evaporation. Oneespecially suitable organic solvent is methanol.

The dried and optionally heat treated particles and the dedusting agentshould be contacted under conditions such that the particles can becoated with the dedusting agent, but such that the dedusting agent doesnot significantly diffuse into the internal structure of the particles.Preferably, such contacting will be conducted with some form ofmechanical distribution, such that adequate distribution of thededusting agent on the water-absorbent resin particles occurs. Examplesof blending equipment/processes include simple tumbling of a jar, orblending in a conical dryer, ribbon blender, drum tumbler, etc. Moderatestirring, shaking, or even a short distance of conveying in ascrew-conveyer can be sufficient for such adequate distribution of thededusting agent over the particles, particularly if the particles are atan elevated temperature. Moderate grinding will also suffice, but is notnecessary.

The temperature of contacting can be any temperature at which thededusting agent does not significantly react with the carboxyl moietiesof the absorbent resin polymer or evaporate. Such temperatures aretypically at least from about 20 to about 150° C. It should be notedthat elevated temperatures, i.e., those above ambient temperatures,improve the speed of coating of the particles.

While not necessary, the dedusted compositions may be further blendedwith a particulateflowabilityenhancer,suchasfumedsilica.Onesuchsilicaisanultra-microscopic silica having a BET surface area ofat least 50 m² /g and a particle diameter less than or equal to 0.05micron, such asAEROSIL™ 200 (available from Degussa Inc.). Whenutilized, the particulate flowability enhancer will be provided in anamount less than 10 weight percent, more typically less than 1 weightpercent, based on the weight of the dedusted composition.

The subject invention provides a process for measuring the amount ofunassociated matter having a diameter less than a predetermined sizepresent in a water-swellable polymer composition. in the case of theExamples, the measurement process was used to determine the amount ofunassociated matter having a maximum size less than or equal to 10microns. The process is applicable, however,to measure the amount ofunassociated matter having a diameter less than other predeterminedsizes, the predetermined sizes typically consistent with either themanufacturer's or the purchaser's specifications. The inventive processcomprises:

(a) placing the composition into the sample holder of a pulsed jetdisperser;

(b) blasting the composition with air dried by passage throughdehydration means; and

(c) determining the weight of particles in each of a plurality of sizeincrements.

Dehydration means are suitable to reduce the amount of water present inthe air to be used to blast the polymer composition. Suitabledehydration means include molecular sieve cartridges, dessicatingmaterials, membranes, etc. An especially preferred dehydration means isa molecular sieve cartridge.

The dust levels of the coated samples may be measured as follows.Approximately 0.01 to 0.2 grams of the sample to be tested is placed inthe sample holder of an AEROSIZER™ pulsed jet disperser, available fromAmherst Process Instruments (Hadley, Massachusetts). The pulsed jetdisperser is fitted with a 250 micron screen to prevent particlesgreater than 250 microns in diameter from entering the detector. The airdelivered to the pulsed jet disperser is filtered using the factorysupplied filter and is dried using a molecular sieve cartridge. Thesamples are blasted with the dried air, with the exhaust stream beingdirected to the detector and analyzed until the instantaneous count ratedecays to less than 20 particles per second on the low sensitivitysetting of the detector.

Measurements are made on separate portions of the samples and areaveraged. For each portion, a determination is made of the total numberof particles in each of 500 size increments evenly distributedlogarithmically from 0.2 to 200 microns, the instrument exhibiting anapproximate lower level of detection of 0.5 microns. The weight ofmaterial in each of these size increments is calculated using thefollowing formula (1):

    Weight=(number of particles in an increment)(density)(π)(particle diameter.sup.3)/6.                                        (1)

For sodium polyacrylate superabsorbent materials, the density is assumedto be 1.60 g/cm³.

The aggregate weight of unassociated material less than 10 microns isdesignated weight_(dust). This weight is compared to the weight ofsample originally introduced into the disperser, i.e., weight_(sample).The percent dust is determined in accordance with equation (2):

    % dust=100(weight.sub.dust)/(weight.sub.sample).           (2)

The following examples are provided for the purpose of explanationrather than limitation. It will be noted that the water-swellablepolymer samples are standard commercially available materials. Theactual composition of these materials is not relevant to the presentinvention, i.e., the invention would be expected to be applicable to anycommercially available water-swellable polymer materials.

EXAMPLE ONE

A sample of a water-swellable polymer is obtained. The sample utilizedis a 62 percent neutralized polyacrylate crosslinked with allylmethacrylate, which sample was fabricated from a monomer mixturecontaining 10 weight percent fine particles having an average diameterless than 140 mesh, and which sample was heat treated at a temperatureof 200° C. As set forth in Tables One and Two, the indicated dedustingagent is added dropwise to the polymer in the indicated amount. Thecoated particles are then rolled for one hour to facilitate distributionof the dedusting agent on the particle surfaces.

The data collected with respect to the sample, as treated with a varietyof dedusting agents, is set forth in Table One.

                  TABLE ONE                                                       ______________________________________                                                              Unassociated                                                                             Percent                                          dust reduction in                                                           Sample Dedusting Agent (ppm) dust                                           ______________________________________                                        1       none(control) 7.10       --                                               8.20                                                                          8.90                                                                          Avg: 8.07                                                                   1A      l wt. percent  none detected 100                                        VORANOL ™ 2070                                                             polyether polyol                                                            1B 0.25 wt. percent  none detected 100                                         VORANOL ™ 2070                                                             polyether polyol                                                             1C 0.25 wt. percent  0.0046 >99                                                glycerin  0.00055                                                              0.0061                                                                        Avg: 0.0038                                                                 1D  l wt. percent  0.012 >99                                                   glycerin  0.0048                                                               0.0048                                                                        0.0013                                                                        Avg: 0.0057                                                               ______________________________________                                    

EXAMPLE TWO

A sample of a water-swellable polymer is obtained. The sample utilizedis a 62 percent neutralized polyacrylate crosslinked withtrimethylolpropane triacrylate. The sample is screened to a particlesize between 20 and 325 mesh. As set forth in Table Two, the indicateddedusting agent is added dropwise to the polymer in the indicatedamount. The coated particles are then rolled for one hour to facilitatedistribution of the dedusting agent on the particle surfaces.

The data collected with respect to the sample, as treated with a varietyof dedusting agents, is set forth in Table Two.

                  TABLE TWO                                                       ______________________________________                                                                          Percent                                         Unassociated dust reduction in                                              Sample Dedusting Agent (ppm)  dust                                          ______________________________________                                        2      none (control) 7.03        --                                              20.3                                                                          17.4                                                                          9.6                                                                           Avg: 14                                                                     2A  l00 ppm 0.84 91                                                            VORANOL ™ 2070  0.82                                                       polyether polyol         2.2                                                    Avg: 1.3                                                                   2B 300 ppm  0.087  99                                                          VORANOL ™ 2070  0.071                                                      polyether polyol        0.21                                                     Avg: 0.12                                                                 2C  l000 ppm 0.12  >99                                                         VORANOL ™ 2070 0.049                                                       polyether polyol  0.21                                                         Avg: 0.082                                                                  2D 3000 ppm 0.15 99                                                            VORANOL ™ 2070  0.16                                                       polyether polyol 0.11                                                           Avg: 0.14                                                                  2E 300 ppm   3.7 >99                                                           DOWANOL ™ TPM  1.7                                                         tripropylene  9.1                                                             glycol methyl 2.3                                                             ether Avg: 4.2                                                               2F  3000 ppm.  0.51 >99                                                        DOWANOL ™ 1.1                                                              TPnB tripropylene   0.38                                                      glycol n-butyl      Avg: 0.66                                                 ether                                                                        2G  3000 ppm 31  99                                                             DOWANOL ™ PPH  0.87                                                        polyether alcohol    7.3                                                      propylene glycol     11                                                       phenyl ether Avg: 13                                                      ______________________________________                                    

EXAMPLE THREE

In accordance with Example One, a 62 percent neutralized polyacrylatecrosslinked with allyl methacrylate, which sample was heat treated at atemperature of 200° C. and which was a sample was screened to a particlesize between 20 and 325 mesh is treated with the amount of VORANOL™ 2070polyether polyols as indicated in Table Three. About 10 grams of thetreated polymer is added to an 8 ounce jar which is approximatelyhalf-filled with ceramic rod segmentswhich are about 7 mm long and about7 mm in diameter. The jar is taped and is rolled on a roll mill for theperiod of time indicated in Table Three. Upon completion of milling, theballs are separated from the polymer sample using an 1/8 inch (3 mm)mesh screen. The amount of unassociated dust is measured as describedabove.

                                      TABLE THREE                                 __________________________________________________________________________     Amountof                                                                       VORANOL ™ 2070                                                             polyether polyol Amount of unassociated dust after the indicated number                   of minutes of miIIing (ppm)                                     Sample                                                                            (ppm)     0 min.   5 min.                                                                              10 min.                                                                             l5 min.                                                                              20 min.                             __________________________________________________________________________    3A  none      6.84     5.00        154    532                                    (Control)  6.12  72.2    83.0  638                                            9.86  41.9     138  688                                                        6.75 76.7     139  582                                                             63.8     156                                                             Avg. 7.39  Avg: 51.9    Avg: 134  Avg: 610                                  3B 100  0.280    5.09                                                            0.800        75.0                                                             1.23       97.0                                                              0.960    49.0                                                                 Avg. 5.78     Avg: 55.7                                                     3C 300  0.00910  2.12   74.8                                                    0.0210   3.12                        95.7                                     0.0320   3.92            60.9                                                    7.98                        130                                              14.1                        181                                               11.8                        155                                               4.97                                                                          12.1                                                                        Avg. 0.0207   Avg: 7.51   Avg: 116                                          3D 1,000    0.0180  0.160   7.3                                                 0.00620                      1.01                         15                  0.0140                       1.08                        32                   0.0490                      0.900               9.9                              3.20                        9.7                                              10.8                                                                        Avg. 0.0218   Avg: 1.2    Avg: 14.8                                       __________________________________________________________________________

As illustrated by the data set forth in Table Three, the application ofthe dedusting agent to the polymer improves the resistance of thepolymer to attrition upon handling. In particular, a preferred inventivecomposition treated with 1,000 ppm VORANOL™ 2070 polyether polyolcomprises no more unassociated dust upon roll milling for 10 minutes(1.24 ppm) than does an untreated unmilled sample (7.39 ppm). Further, apreferred inventive composition treated with 3,000 ppm VORANOL™ 2070polyether polyol comprises only about twice as much unassociated dustupon roll milling for 20 minutes (14.7 ppm) than does an untreatedunmilled sample (7.39 ppm); with the composition treated with 3,000 ppmVORANOL™ 2070 polyether polyol comprising less than 3 percent of theamount of unassociated dust present in an untreated sample which hasbeen roll milled for 20 minutes (14.7 ppm, as compared to 610 ppm).

As further illustrated by the data set forth in Table Three, a preferredinventive composition treated with 1,000 ppm VORANOL™ 2070 polyetherpolyol comprises less than 35 ppm unassociated dust having a maximumdiameter less than or equal to 10 microns upon being roll milled for 20minutes in accordance with the milling procedure described in thisExample, as compared with 610 ppm for an untreated sample. Further,another preferred inventive composition treated with at least 300 ppmVORANOL™ 2070 polyether polyol comprises no more than 15 ppmunassociated dust having a maximum diameter less than or equal to 10microns upon being roll milled for 10 minutes in accordance with themilling procedure described in this Example, as compared with what isexpected to be about 100 ppm for an untreated sample.

Moreover, the application of the dedusting agent to the polymer samplesdoes not appear to detrimentally affect the performance or properties ofthe polymer. For instance, the flow time, bulk density, centrifugecapacity, 0.3 psi (21,000 dynes/cm²) AUL and 60 minute extractableslevels of samples 3A, 3B, 3C, and 3D are set forth in Table Four.

Flow time and bulk density are measured in accordance with ASTM D1895-69; sixty minute 0.3 psi (21,000 dynes/cm²) AUL is measured asdescribed in U.S. Pat. No. 5,147,343; centrifuge capacity is determinedas described in U.S. Pat. No. 4,286,082 except that an absorption timeof 30 minutes rather than 3 to 5 minutes is employed; and the 60 minuteextractables level is determined as described in U.S. Reissue Patent32,649, with the relevant portions of each of the citedprocedures/references being incorporated herein by reference.

                  TABLE 4                                                         ______________________________________                                                                 Centri-                                                  Bulk fuge 0.3 psi 60 minute                                                  Flow time Density Capacity AUL extract-                                      Sample (sec) (g/mL) (g/g) (g/g) ables (%)                                   ______________________________________                                        3A    9.3       0.66     29.0   29.8   3.9                                      3B      8.1     0.72     28.6      31.9    3.8                                3C      8.0     0.72     28.7      30.9    4.1                                3D      8.4     0.70     28.5      30.1    4.0                              ______________________________________                                    

In view of the description and examples, other embodiments will bereadily ascertained by one having skill in the art. Accordingly, thescope of the invention shall be limited only by the claims set forthbelow.

We claim:
 1. A composition comprising free-flowing water-absorbentlightly crosslinked water-insoluble superabsorbent polymer particles,the particles bearing on their surface from about 100 ppm to 3000 ppm,based on the weight of the particles, of a physical coating of a liquidhydrophilic organic dedusting agent, wherein the composition comprisesno more than 2.5 ppm unassociated dust having a maximum diameter lessthan or equal to 10 microns, and wherein the composition ischaracterized by a 60 minute 0.3 psi (21,000 dynes/cm²) Absorption underLoad greater than 20 grams/gram, and wherein the composition issubstantially free of inorganic powder.
 2. The composition of claim 1wherein the composition comprises no more than 35 ppm unassociated dusthaving a maximum diameter less than or equal to 10 microns upon beingroll milled for 20 minutes.
 3. The composition of claim 1 wherein thecomposition comprises no more than 15 ppm unassociated dust having amaximum diameter less than or equal to 10 microns upon being roll milledfor 10 minutes.
 4. The composition of claim 1 wherein the dedustingagent is a polyol, an ethoxylated surfactant, or a nonionic surfactanthaving an HLB value of at least
 7. 5. The composition of claim 4 whereinthe dedusting agent is a polyether polyol, a polyethylene glycol orglycerol.
 6. The composition of claim 1, wherein the dedusting agent isprovided in an amount of from about 300 ppm to 1,000 ppm, based on theweight of the particles.
 7. The composition of claim 6 wherein thededusting agent is a polyether polyol.
 8. The composition of claim 6wherein the dedusting agent is a polyethylene glycol.
 9. The compositionof claim 6 wherein the dedusting agent is glycerol.
 10. A compositionconsisting essentially of a free flowing water-absorbent lightlycrosslinked water-insoluble partially neutralized polyacrylic acidpolymer particles, the particles bearing on their surface from about 300ppm to 1,000 ppm, based on the weight of the particles, of a liquidhydrophilic organic dedusting agent, wherein the composition comprisesno more than 2.5 ppm unassociated dust having a maximum diameter lessthan or equal to 10 microns, and wherein the composition ischaracterized by a 60 minute 0.3 psi (21,000 dynes/cm²) Absorption underLoad greater than 20 grams/gram and wherein the composition issubstantially free of inorganic powder.
 11. A process for thepreparation of free-flowing water-absorbent lightly crosslinkedwater-insoluble polymer particles comprising:(a) preparing awater-swellable hydrogel by a gel polymerization process; (b) drying andsizing the hydrogel to form a composition comprising dried and sizedparticles, the composition comprising a first amount of unassociateddust having a maximum diameter less than or equal to 10 microns; and(c)contacting the composition with from about 100 ppm to 5000 ppm, basedon the weight of the particles, of a physical coating of a hydrophilicliquid organic dedusting agent, which is substantially free of inorganicpowder, to form a dedusted composition, wherein the dedusted compositionis characterized by a 60 minute 0.3 psi (21,000 dynes/cm²) Absorptionunder Load greater than 20 grams/gram and wherein the dedustedcomposition comprises a second amount of unassociated dust having amaximum diameter less than or equal to 10 microns, the second amountbeing less than about 10 percent of the first amount.
 12. The process ofclaim 11, wherein the second amount is less than about 2.5 ppm.
 13. Theprocess of claim 12, wherein the second amount is less than about 1 ppm.14. The process of claim 13, wherein the second amount is less thanabout 0.2 ppm.
 15. The process of claim 11 wherein the composition ischaracterized by the tendency to generate a second amount ofunassociated dust having a maximum diameter less than or equal to 10microns upon roll milling of the attrition resistant composition for 10minutes, the second amount being less than or equal to the first amount.16. The process of claim 11, wherein the dedusting agent is a polyol, anethoxylated surfactant, or a nonionic surfactant having an HLB value ofat least
 7. 17. The process of claim 16, wherein the dedusting agent isa polyether polyol, a polyethylene glycol or glycerol.
 18. The processof claim 17 wherein the dedusting agent is provided in an amount fromabout 300 ppm to 3000 ppm, based on the weight of the particles.
 19. Theprocess of claim 17 wherein the dedusting agent is a polyether polyol.20. The process of claim 17 wherein the dedusting agent is apolyethylene glycol.